Thank you. Roman, you're right, that doesn't work anymore. It will always return
I found another solution, which seems to work fine so far:
var cart = ShoppingCartInfoProvider.GetShoppingCartInfo(orderObj.OrderCompletedByUserID, "SITENAME");
I also thought about using the CustomData property instead, however, I like having the fields defined on the class and the option to display those fields in the Kentico backend user interface.
Now I just have to find out how to set those fields when creating an Order directly in Kentico instead of the MVC application.